1. Field of the Invention
This invention relates to printing, more specifically to printing with enhanced images and gray scale.
2. Background of the Invention
Digital copiers and printers have inherent problems reproducing high-resolution images and gray scale. Desktop laser printers generally have only 300 dots-per-inch (dpi) addressability, and print with slightly oversized pixels to allow overlap, which defines the actual printer resolution. These limitations show up as ragged edges on text and aliasing artifacts on graphics because at nominal resolutions the individual pixels cannot accurately replicate curves and diagonal lines.
An alternate dot manipulation method has appeared commercially to address the drawback, but it does not produce gray scale levels. Instead, it varies the shape, size, or location of digital black pixels by controlling laser power and timing. Shortening the dot exposure time, and delaying it, creates a smaller, oval spot which allows dot positioning within the standard pixel cell boundaries. Additionally, the spot can move across the width of the pixel cell in the direction of the raster scanning motion. Reducing power levels in the laser can flatten the spot in the print-process direction orthogonal to the scanning motion. Typically the scan direction is horizontal and the print-process direction is vertical on a page. Smoother appearing edges are achieved by appropriate placement of suitable distorted pixels, and the method is generally referred to as resolution enhancement.
Laser printers can readily achieve higher resolutions than 300 dpi with more complex controllers and printer process subsystems. Printer page-description languages, for example, can present a document, or image, to a digital printer at its limiting resolution. However, the burdens on printer memory, microprocessors, and the capability of the printer equipment and optical scanner to support higher resolutions typically climb as a square of the linear resolution. Systems used in applications demanding higher resolution can run from 1200 to 2500 dpi, but they are proportionately more expensive than generic desktop 300 dpi printers. The benefit is that the added linear resolution permits a binary printer to simulate gray scale images through a process called half-toning.
The xerographic process as embodied in printers, copiers and facsimiles, is binary in nature, making it difficult to achieve varying shades of gray. The development process, in which charged toner particles are attracted to the latent image exposed on a photoreceptor, operates as if it were digital in nature, (i.e., it is a very high contrast analog process). Therefore it is necessary to use higher resolution binary xerographic systems that resort to a method called half-toning to simulate gray scale. Smaller pixels are progressively clustered to form a larger pixel, or half-tone cell. This allows a varying of the number and arrangement of the elements that are to be white or black, resulting in a visually "gray" half-tone cell.
The precision and computational power to generate such a cell is much higher than for a binary desktop laser printer and represents a limiting factor in a laser printer's ability to achieve gray scale. In addition to higher perceived resolution, gray scale is rapidly becoming a necessary feature in a printer designed to reproduce scenic images from photographic sources, or computer displays, because of the inherent complexity of the half-toning process.
In the latter case, computer displays can take advantage of the integrating response of the human eye to vary the gray scale or intensity of an image over a wide dynamic range. Pixels can turn off for periods of time within a frame's display time that the eye integrates together to produce a perceived continuous tone intensity gradation. Since computer displays frequently provide the images for printing, an incompatibility between the image produced on a display and the ability of a digital, binary printer to print it exists. The same is true for digitally scanned continuous-tone photographic images.
In summary, printing with binary digital printers has limitations. There exist difficulties in reproducing characters without distortion, loss of detail, sampling artifacts, or positional errors at lower resolutions. Gray scale simulation using higher resolution printers, must result in acceptable combinations of gray and freedom from visual artifacts.